旋流导风板湿式冷却塔填料区热力性能的数值研究

针对环境侧风对湿式冷却塔填料区传热传质性能的重要影响,基于ANSYS FLUENT软件对600 MW机组湿式冷却塔内的流场进行数值模拟,研究了加装旋流型导风板对填料区热力性能的影响,并对比分析了不同旋流叶片弧度(φ=0、π/12、π/6、π/4)对填料区热力性能的优化程度。结果表明:当侧风风速从0m/s增至4 m/s时,填料区的传质传热能力恶化,4 m/s时取得填料区传质传热系数极小值;加装旋流型导风板后,4 m/s时的传质系数相较于无侧风时平均降低了27.92%,传热系数相较于无侧风时平均降低了24.44%;ψ=π/6的旋流型导风板的优化效果最佳,在侧风风速为4和6 m/s时,传质系数较之于无导风板工况分别提升了4.37%和11.27%,传热系数较之于无导风板工况分别提升了19.76%和26.93%;当侧风风速为4 m/s时,冷却效率系数与冷却数均最低,这与冷却塔的传热传质性能有关,加装π/6的旋流型导风板,在侧风风速为4 m/s时,两系数较之无导风板工况分别提升了10.32%和53.81%。     

侧风作用下火电厂冷却塔性能的实验研究

冷却塔是火电厂重要设备之一,冷却塔冷却性能的优劣直接影响机组效率与安全.通过建立火电厂冷却水系统实验平台,研究了侧风作用下冷却塔的冷却性能.研究发现:随着环境侧风的增加,冷却温差和效率呈先减小后增大的趋势,拐点风速值为0.8m/s;迎风面的进塔风速随之变大,背风面的通风量减小;并采用弗劳德准则数分析了环境侧风对冷却塔性能的影响.     

环境侧风对300MW火电机组冷却塔影响的数值模拟

以300MW机组双曲线自然通风冷却塔为研究对象,采用Fluent软件对冷却塔进行了三维数值模拟,分析了变工况下环境侧风对冷却塔冷却性能的影响、环境侧风对冷却塔进风口速度场的影响。研究发现:随着侧风风速的增加,冷却温差先减小后增大,出塔水温先增大后减小;并且,环境侧风的存在会扰乱冷却塔进风口流场,其临界风速值为6m/s。     

侧风对太阳能增强型冷却塔排热量影响的数值模拟与分析

以一种新型空气冷却系统——太阳能增强型冷却塔为研究对象,通过数值模拟,研究侧风风速对其内部空气动力场、速度场和冷却性能的影响,并与自然通风干式冷却塔进行对比分析,同时对3种典型防风装置及其耦合方式的防风效果进行研究。结果表明:低风速下太阳能增强型冷却塔对侧风的敏感程度低于自然通风干式冷却塔,当风速超过7 m/s后则反之;导流板和挡风板在不同的侧风条件下各具优势;综合考虑导流板与120°分布挡风板的耦合方式是最为有效的防风措施,排热量最大可提高10.6%。     

超大型湿冷塔进风流场及导流装置数模研究及应用

基于Fluent软件,采用标准k-ε湍流模型进行应力封闭,对某工程塔内传热传质过程进行三维数值计算。计算分析了塔内外空气的速度场、温度场,建立了相关方程及气水两相间传热传质理论模型。结合工程实际情况,对冷却塔进风流场进行深入分析:1)导风板的存在降低了塔侧空气绕流流速,增大了冷却塔进风口流场的对称性,使塔内空气动力场的均匀程度增加;导风板安装高度和长度对冷却塔进风流场产生较大影响,以高11 m、长8 m导风板对# 1、# 2冷却塔性能的改善作用最大;2)导风板安装角度和块数对冷却塔进风流场产生一定影响,在导风板安装角度由0°至20°变化、在导风板安装块数由60块至90块变化时,两塔冷却性能变化影响较小。     

湿式冷却塔加装挡风板的数值研究

针对我国北方冬季湿式冷却塔运行时填料下表面、进风口以及基环面等处容易结冰的问题,在冷却塔的进风口处加装挡风板,建立了冷却塔内的传热传质模型,并采用CFD软件模拟和分析了在不同横向风速和不同环境温度下加装不同层数的挡风板时冷却塔的热力特性.结果表明：在风速小于4m/s时,塔内的迎风面空气温度较低,极易结冰;随着风速的增大,低温区逐渐向背风侧转移;当风速为8m/s,环境温度分别为-10℃、-17℃、-23℃时,分别在冷却塔内加装1层、3层和5层挡风板,能大大改善塔内温度场,有效防止塔内结冰.     

湿式冷却塔进风口加装导风板的优化设计

基于两相流传热传质理论,利用Fluent软件模拟300 MW机组冷却塔填料区使用多孔介质时的通风率,采用离散相模型（DPM）在配水区上表面加入热水,模拟研究新型旋流型叶片导风板的优化能力,给定不同弧度及安装角,分别在0、3和7 m·s^-1风速下计算冷却塔出塔水温,并分析侧风对冷却塔冷却性能的影响。研究结果表明：加装导风板可以降低侧风引起的不利影响,导风板数量为50块时效果最好,旋流型叶片导风板的最佳安装角为20°,此时旋流型叶片的最佳弧度为15°,最大温降可达0.787 4 K。研究结果为火电厂选择导风板提供了依据。     

无级弧线型挡板对火电机组冷却塔性能影响的实验

建立300 MW火电机组实验模型,研究弧形挡板对冷却塔性能的影响,获得最佳弧形挡板布置方式,并对各因素进行回归分析,找出冷却塔高效运行的工况范围。研究表明:存在临界风速vcr,当风速小于该值,随风速增大冷却性能逐渐降低;当风速大于该值,随风速增大冷却性能逐渐提高;弧形挡板可有效减弱环境侧风对冷却性能的不利影响,挡板安装角在45°~75°范围内冷却温差和效率相较于无侧风无弧形挡板都有不同程度的提高;进塔水温(28~38℃)越高时,冷却塔效率越高。     

双层布置间冷散热器数值试验研究

利用ANSYS Fluent软件，在自然通风状态下，对我国首创并实施的双层布置间冷散热器的流动和换热性能进行了数值模拟、分析和研究。环境风速不大于4.5m/s时，下层各冷却柱的通风量和换热量均高于相应上层冷却柱。当环境风速为10m/s～20m/s时，间冷塔迎风面，下层冷却柱的通风量和换热量均低于上层；在间冷塔两侧及下风侧，下层冷却柱的通风量和换热量则高于上层。随着环境风速的增大，上层冷却柱将首先出现热空气流出间冷塔和“穿堂风”的现象。不同环境风速下，下层冷却柱的总通风量和总换热量均高于上层，二者总通风量相差1.29%～10.18%,总换热量相差1.11%～9.23%。     

不同翅墙安装角度对间接空冷塔内外流动传热性能的影响

针对SCAL型300 MW间接空冷系统空冷散热器的典型结构,建立了安装十字翅墙前后的空冷塔三维流动传热物理模型,并利用CFD和多孔介质模型模拟获得了间接空冷塔内外空气的流场和温度场,进而研究了不同环境风速影响下安装不同安装角度十字翅墙前后空冷塔内外空气流动传热特性的变化规律。研究表明:安装0°和45°翅墙能有效削弱环境横向风的不利影响,改善空冷散热器的传热状况。其中,0°翅墙对6 m/s~9 m/s风速下的空冷塔性能改善比较明显,45°翅墙主要改善4 m/s~7 m/s风速下的空冷塔性能。     

Performance prediction of wet cooling tower using artificial neural network under cross-wind conditions

This paper describes an application of artificial neural networks (ANNs) to predict the thermal performance of a cooling tower under cross-wind conditions. A lab experiment on natural draft counter-flow wet cooling tower is conducted on one model tower in order to gather enough data for training and prediction. The output parameters with high correlation are measured when the cross-wind velocity, circulating water flow rate and inlet water temperature are changed, respectively. The three-layer back propagation (BP) network model which has one hidden layer is developed, and the node number in the input layer, hidden layer and output layer are 5, 6 and 3, respectively. The model adopts the improved BP algorithm, that is, the gradient descent method with momentum. This ANN model demonstrated a good statistical performance with the correlation coefficient in the range of 0.993–0.999, and the mean square error (MSE) values for the ANN training and predictions were very low relative to the experimental range. So this ANN model can be used to predict the thermal performance of cooling tower under cross-wind conditions, then providing the theoretical basis on the research of heat and mass transfer inside cooling tower under cross-wind conditions.     

Forced convective boiling of subcooled water at high velocity

This paper deals with heat transfer and critical heat flux (CHF) in subcooled flow boiling offering a fundamental study aimed at high heat flux cooling. Experiments with water at 0.12 MPa were conducted in a mass velocity range from 500 kg/m²s to 15,000 kg/m²s (velocity from 0.5 m/s to 15 m/s) and subcooling from 20 K to 60 K. A sheet of stainless steel (80 mm in heated length, 10 mm wide, and 0.2 mm thick) was mounted flush with a sidewall of a vertical rectangular channel (cross-section 20 mm by 30 mm) and heated directly using direct current. It was found that mass velocity and subcooling strongly affect CHF and heat transfer in non-boiling convection and partial nucleate boiling regimes. These two parameters have no appreciable influence in the fully developed nucleate boiling regime. In the parameter range used, CHF reached 15 MW/m². Boiling bubble behavior just prior to reaching CHF was found to vary depending on mass velocity and subcooling. 1998 Scripta Technica, Heat Trans Jpn Res, 27(5): 376–389, 1998     

Experimental investigation into the thermal-flow performance characteristics of an evaporative cooler

This study investigates the thermal-flow performance characteristics of an evaporative cooler. The derivation of the Poppe [1] and Merkel [2] analysis for evaporative coolers are presented and discussed. Performance tests were conducted on an evaporative cooler consisting of 15 tube rows with 38.1 mm outer diameter galvanized steel tubes arranged in a 76.2 mm triangular pattern. From the experimental results, correlations for the water film heat transfer coefficient, air–water mass transfer coefficient and air-side pressure drop are developed. The experimental tests show that the water film heat transfer coefficient is a function of the air mass velocity, deluge water mass velocity as well as the deluge water temperature, while the air–water mass transfer coefficient is a function of the air mass velocity and the deluge water mass velocity. It was found that the correlations obtained for the water film heat transfer coefficient and the air–water mass transfer coefficient compare well with the correlations given by Mizushina et al. [3]. Relatively little published information is available for predicting the air-side pressure drop across deluged tube bundles. The present study shows that the pressure drop across the bundle is a function of the air mass velocity and the deluge water mass velocity.     

Numerical Analyses of Single‐Phase Pressure Drop and Forced Convective Heat Transfer Coefficient of Water–Ethanol Mixture: An Application in Cooling of HEV Battery Module

The present numerical analyses are related to the cooling of a hybrid electric vehicle (HEV) battery module by water–ethanol mixture. The fluid is passed through a cold plate consisting of two rectangular channels of 0.01 m depth, 0.015 m width, and 0.15 m length. The battery module is represented by a heater placed below the cold plate. The single‐phase pressure drop and single‐phase heat transfer coefficient for water, water–ethanol mixture of mass fraction of 25%, 50%, and 75%, and ethanol are determined numerically for different heat fluxes of 10, 15, 20, and 25 kW/m² and different Reynolds numbers 500, 1000, 1500, 2000, and 2500. To solve the Navier–Stokes equation, the pressure correction method was used and to solve the energy equation, the Lax–Wendroff explicit method is used. Numerical results obtained for water are compared with the literature correlations. The friction factor for water deviated by an average of 8.02% from the Lewis and Robertson equation. The Nusselt number for water deviated by 7.35% from the Churchill and Ozoe equation at lower Reynolds number 500 and at higher Reynolds number 2500, Nusselt number deviated by 13.68% from the Stephan equation. The results showed that the heat transfer coefficient increased with an increase in Reynolds number and heat flux. The effect of the increase in Reynolds number is more significant than the increase in heat flux. At higher ethanol mass fraction and higher Reynolds number the heat transfer coefficient increased with heat flux when compared to water. There is no significant decrease in heat transfer coefficient with an increase in ethanol mass fraction. The pressure drop increased and the heat transfer coefficient decreased with an increase in ethanol mass fraction.     

Studies on gas–solid heat transfer during pneumatic conveying

Interactions between solids and gas during pneumatic conveying can be utilized for variety of applications including flash drying, solids preheating etc. Experiments on air–solid heat transfer were carried out in a vertical pneumatic conveying heat exchanger of 54 mm inside diameter, using gypsum as the solid material. The effect of solids feed rate (0.6–9.9 g/s), air velocity (4.21–6.47 m/s) and particle size (231–722.5 μm) on air–solid heat transfer rate, heat transfer area and air–solid heat transfer coefficient has been studied. Empirical correlations have been proposed for the prediction of Nusselt number based on the present experimental data. The proposed correlations predict Nusselt number within an error of ±15% for the present data.     

Numerical simulation of a closed wet cooling tower with novel design

A closed wet cooling tower with novel design was proposed and numerically investigated. The studied cooling tower consists of two main parts: one heat and mass transfer unit (HMTU) and one heat transfer unit (HTU). In the HMTU, copper tubes are arranged as heat transfer tubes while plastic tubes are collocated to enlarge the mass transfer area between the spray water and the airflow. In the HTU, only copper tubes are adopted as heat transfer tubes. Heat and mass transfer process takes place among the process water, airflow and spray water in the HMTU, while in the HTU only heat transfer between the process water and the spray water is observed. A transient one dimensional distributed-parameter model was adopted to evaluate the cooling tower performance under different operating conditions. Determination of heat and mass transfer coefficients, as well as the influence of Lewis number on the cooling tower performance, was presented.     

Effect of unequal fuel and oxidizer Lewis numbers on flame dynamics

The interaction of non-unity Lewis number (due to preferential diffusion and/or unequal rates of heat and mass transfer) with the coupled effect of radiation, chemistry and unsteadiness alters several characteristics of a flame. The present study numerically investigates this interaction with a particular emphasis on the effect of unequal and non-unity fuel and oxidizer Lewis numbers in a transient diffusion flame. The unsteadiness is simulated by considering the flame subjected to modulations in reactant concentration. Flames with different Lewis numbers (ranging from 0.5 to 2) and subjected to different modulating frequencies are considered. The results show that the coupled effect of Lewis number and unsteadiness strongly influences the flame dynamics. The impact is stronger at high modulating frequencies and strain rates, particularly for large values of Lewis numbers. Compared to the oxidizer side Lewis number, the fuel side Lewis number has greater influence on flame dynamics.     

Effect of probe size on heat transfer at the wall in circulating fluidized beds

An experimental investigation was made to study the effect of vertical probe height on heat transfer at the wall in circulating fluidized beds. Experiments were conducted in a 100 mm internal diameter, 5.15 m tall circulating fluidized bed. Four probes having 85, 127.5, 170 and 255 mm heights were tested. Heat transfer measurements covered a range of superficial velocity from 7.2 to 12.5 m/s and a range of suspension density from 25 to 68 kg/m³. The results were compared with those of other investigators. An empirical correlation incorporating the dimensionless probe height and the particle Nusselt number and Reynolds number has been suggested.     

A Mathematical Study for Laminar Boundary‐Layer Flow,Heat, and Mass Transfer of a Jeffrey Non‐Newtonian Fluid Past a Vertical Porous Plate

In this article, we investigate the nonlinear steady‐state boundary‐layer flow, heat and mass transfer of an incompressible Jeffrey non‐Newtonian fluid past a vertical porous plate. The transformed conservation equations are solved numerically subject to physically appropriate boundary conditions using a versatile, implicit finite‐difference technique. The numerical code is validated with previous studies. The influence of a number of emerging non‐dimensional parameters, namely, Deborah number (De), Prandtl number (Pr), ratio of relaxation to retardation times (λ), Schmidt number (Sc), and dimensionless tangential coordinate (ξ) on velocity, temperature, and concentration evolution in the boundary layer regime are examined in detail. Furthermore, the effects of these parameters on surface heat transfer rate, mass transfer rate, and local skin friction are also investigated. It is found that the velocity is reduced with increasing Deborah number whereas temperature and concentration are enhanced. Increasing λ enhances the velocity but reduces the temperature and concentration. The heat transfer rate and mass transfer rates are found to be depressed with increasing Deborah number, De, and enhanced with increasing λ. Local skin friction is found to be decreased with a rise in Deborah number whereas it is elevated with increasing λ. And an increasing Schmidt number decreases the velocity and concentration but increases temperature. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21111     

A modified logarithmic mean enthalpy difference (LMED) method for evaluating the total heat transfer rate of a wet cooling coil under both unit and non-unit Lewis Factors

The logarithmic mean enthalpy difference, (LMED) method has been extensively used in evaluating the thermal performance of an air cooling coil under wet condition. The LMED method has been developed based on the assumption of unit Lewis Factor, i.e., Le^2/3 = 1. However, a number of previous studies have suggested that the Lewis Factor can actually deviate from being 1. Consequently, errors can be resulted in when calculating the total heat transfer rate of a wet cooling coil using the LMED method. Therefore, a modified LMED (m-LMED) method has been developed for calculating the total heat transfer rate under both unit and non-unit Lewis Factors and is reported in this paper. This m-LMED method has been validated by comparing its prediction of the total heat transfer rate to that from numerically solving the fundamental governing equations of heat and mass transfer of a wet cooling coil. The m-LMED method can therefore replace the LMED method for calculating the total heat transfer rate of a wet cooling coil under both unit and non-unit Lewis Factors.